Various polymers comprising photochromic groups are known from the literature: the special peculiarity thereof is that their optical properties, such as absorption, emission, reflection, birefringence and scattering, can be induced by light and can be varied reversibly. Polymers of this type have a special branched structure: side groups which are capable of absorbing electromagnetic radiation are seated—via parts of molecules which act as spacers—on a linear backbone. The interest of experts in this field has recently been directed towards side group polymers such as these which comprise side groups of different types, one type of which is capable of absorbing electromagnetic radiation whilst the other type is a mesogenic group, the shape of which is anisotropic. Liquid crystalline side group polymers of this type are described in U.S. Pat. Nos. 4,631,328 and 4,943,617, for example. In their unoriented state, films of these polymers are turbid and scatter light; these films do not become clear and transparent until they are aligned.
Amorphous polymers which are suitable for storing optical information are known from DE-OS 38 10 722 and U.S. Pat. No. 5,173,381. These have the technical advantage that films made of these polymers exhibit usable optical properties immediately after they are produced.
Homopolymers are seldom mentioned in this connection. EP-A 617 110 describes azo-containing carbaminates which are rendered polymerisable by N-acylation with (meth)acrylic acid. In actuality, homopolymers are generally inferior to copolymers.
The only processes which have been described hitherto for the reversible storage of information are those where deletion of the information is effected by raising the temperature, and can be effected both by heat and by light. Moreover, deletion by light can exhibit the advantage that the process is limited to a defined location, which is why this variant is preferred. In general, it can be stated that on raising the temperature the property of retaining stored information is lost. The known compounds therefore have the disadvantage that the birefringence effects which are written in are not thermally stable. At elevated temperatures, particularly at temperatures approaching the glass transition temperature, birefringence becomes less pronounced and finally disappears completely. There is therefore a need for information storage media for which the stability of written information is as insensitive to temperature as possible.